The present invention relates in general to instruments for measuring material properties and, more particularly, to an improved instrument for on-line measurement of material properties by scatter-mode multi-wavelength differential absorption spectroscopy.
U.S. Pat. No. 5,338,361 discloses a sensor for on-line coating measurements of a moving paper sheet. The sensor includes a light source for transmitting a beam of infrared radiation toward the moving sheet. The infrared radiation encompasses a broad range of wavelengths. Different coatings, such as clay and latex, as well as moisture absorb different wavelengths in varying amounts. In one embodiment, a receiver is located on the same side of the moving sheet as the source for measuring the intensity of the reflected portion of the beam radiation. The receiver comprises at least one beam splitter and a plurality of separate detectors. A band pass filter is associated with each detector to pass that portion of the spectrum falling within the pass band. Hence, a first band pass filter, associated with a first detector, passes wavelengths of the spectrum which are strongly absorbed by a first coating material, e.g., latex. A second band pass filter, associated with a second detector, passes wavelengths of the spectrum which are strongly absorbed by a second coating material, e.g., clay. A third band pass filter, associated with a third detector, passes wavelengths of the spectrum where the radiation is strongly absorbed by the base sheet, but is only weakly absorbed by the first and second coating components. A fourth band pass filter, associated with a fourth detector, passes wavelengths of the spectrum where the radiation is strongly absorbed by moisture.
A first disadvantage of the sensor disclosed in the '361 patent relates to the long paths from the radiation source to the paper sheet and from the paper sheet to the detectors. These long paths result in a decrease in the total amount of signal received by the detectors. A second disadvantage is that the path lengths to the various detectors are not the same, thereby resulting in the detectors having different fields of view. It is also noted that the reflected radiation should be aligned exactly along the axis of the beam splitter, which is problematic to accomplish.
A similar reflective infrared measuring apparatus is also known comprising a substantially clear window through which a light beam exits before impinging upon a moving substrate. Any dust on the window results in light being back scattered toward one or more radiation detectors. To prevent the scattered light from reaching the radiation detectors, a blocking member is provided in the sensor housing. While the blocking member functions to shield a significant portion of light scattered by dust, the blocking member also shields a portion of the light reflected from the moving substrate from reaching the detectors, which reduces the total sensed radiation scattered back by the substrate reducing the instrument signal-to-noise ratio. Further, because the light emitted by the source passes through a different area of the window as compared to the light scattered back by the substrate, the dust on the window, which is typically non-uniform in distribution, may influence the properties of the light impinging on the substrate differently than the properties of the light back scattered by the substrate. This sensor also suffers from the same first and second disadvantages noted above with regard to the sensor disclosed in the '361 patent.
There is a need for an improved infrared measuring apparatus wherein the path lengths to the detectors are substantially equal in length such that the detectors have substantially the same fields of view and the distances between the light source and the moving substrate and the moving substrate and the detectors are minimized.